This invention relates to a process for the production of methacrylic or acrylic acid by oxidizing methacrolein or acrolein and to a catalyst.
More particularly, this invention relates to a process for the production of methacrylic or acrylic acid by oxidizing methacrolein or acrolein with molecular oxygen in the presence of steam by using a catalyst comprising (1) palladium, (2) phosphorus, (3) antimony, (4) X and (5) oxygen wherein X denotes at least one element selected from the group consisting of potassium, sodium, rubidium, lithium, cerium, beryllium, magnesium, calcium, vanadium, strontium, zinc, thorium and rhenium, and to the catalyst.
The process of the invention may further comprise supplying into the reaction system a phosphoric acid or a phosphorus compound capable of forming a phosphoric acid through a chemical change during the reaction.
For the synthesis of methacrylic acid by oxidizing methacrolein in a vapor phase a number of catalysts have hitherto been proposed.
Almost all of these catalysts, however, have low activities. Further, if the reaction is carried out at an elevated temperatures in order to increase the conversion, large amounts of undesirable by-products such as carbon monoxide, carbon dioxide, acetic acid etc. are produced so that the per-pass yield of methacrylic acid is very low.
The catalysts as disclosed in the Japanese Patent Laid-Open Publication Nos. 67216/1973 and 61416/1973, which are improved in their catalytic activity and selectivity, comprise phosphomolybdic acid or its salts as a main ingredient.
Phosphomolybdic acid-based catalysts have a disadvantage that the catalyst lifetime is short. Once they lose activity, the activity of these catalysts cannot be restored by means of a simple treatment such as re-calcination, for example. When the temperature of reaction or calcination exceeds 450.degree. C., they are abruptly degraded in catalytic activity. Thus, they are thermally unstable and they are not always available for commercial use.
Further, conventional phosphomolybdic acid-based catalysts offer a notably short catalytic lifetime when the reaction is carried out at a particularly high space velocity.
From a commercial aspect, it is desired to develop a catalyst which has acceptable activity and selectivity at low temperatures as well as a longer service life. Particularly desirable is a catalyst which can maintain its activity over a long period of time even when the reaction is carried out at high space velocities.